The technique of supplying power in a non-contact manner is called wireless charging. An example of a wireless charging system includes a power supply device on the power supply side and a power receiving device on the power receiving side, and transmits power through electromagnetic induction or magnetic resonance. Such a wireless charging system has been studied for its application to the transmission of high power used for, for example, charging of an electric vehicle, in addition to the transmission of low power used by, for example, a cellular phone or an electric toothbrush.
The presence of a foreign object which is a conductive material, such as metal, between the power supply device and the power receiving device may cause an eddy current to the foreign object which then may possibly generate heat. When the foreign object is a ferromagnet, such as iron, heat generation may be caused by hysteresis loss. In particular, when the wireless charging system transmits high power, a large amount of heat may be generated by the foreign object.
In view of this, a foreign object detection device has been proposed for the wireless charging system. A known example of the foreign object detection device is a method achieved using a coil. With this method, an electromotive force caused by electromagnetic induction due to a coil placed in a time-varying magnetic field is used. Assume that a foreign object is present and an eddy current passes through this foreign object. In this case, since a part of magnetic flux avoids the foreign object, the magnetic flux density distribution is different from when no foreign object is present. When the foreign object is a ferromagnet, this means that the density of magnetic flux that passes through the foreign object increases. Thus, also in this case, the magnetic flux density distribution is different from when no foreign object is present. In this way, the magnetic field passing through the coil varies depending on the presence or absence of a foreign object. On this account, the electromotive force caused by the electromagnetic induction of the coil also varies depending on the presence or absence of a foreign object. The foreign object detection device detects a foreign object, based on this electromotive force.
A foreign object detection device is known which includes a plurality of coils and in which inductive currents passing through two adjacent coils flow in the mutually opposite directions when no foreign object is present. Moreover, another foreign object detection device is known which detects a foreign object using a magnetic field that is used for power transmission of a wireless charging system (see Patent Literature [PTL] 1)